1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, and more particularly, to an optical scanning device that scans a scanned surface with light and an image forming apparatus that includes the optical scanning device.
2. Description of the Related Art
An image forming apparatus such as a printer or a digital copying machine that uses a laser beam is widely used to record an electrophotographic image. This image forming apparatus includes an optical scanning device. In recording the electrophotographic image, a method that scans a laser beam in an axial direction of a drum having photosensitivity (hereinafter, referred to as “photosensitive drum”) using a deflector (for example, polygon mirror) and rotates the photosensitive drum to form a latent image on the surface (scanned surface) of the photosensitive drum is generally used.
For example, as shown in FIG. 24, a method that forms one pixel by plural light beams is known. In FIG. 24, 12 light emitting portions (a1 to a4, b1 to b4, and c1 to c4) are arranged at an interval of 4800 dpi on an image surface, in a sub-scanning corresponding direction. As shown in FIG. 25, one pixel is formed by 4 light beams at an interval of 1200 dpi on an image surface.
In the image forming apparatus that forms one pixel by plural light beams, the following advantages exist. (1) Since a method that is implemented by the image forming apparatus is superior to a method that forms one pixel by one light beam in terms of the light amount, high-speed write is enabled. (2) Since one pixel can be further divided, smoothness of an inclined line can be secured (jaggy can be decreased), sharpness of a character can be improved, and distortion correction of an image such as correction of bending and inclination based on image processing can be performed.
Meanwhile, since combinations of light emitting portions to emit light beams to form one pixel (hereinafter, simply referred to as “pixel forming light emitting portions”) are sequentially changed, the image density of each pixel is easily changed due to the beam pitch irregularity on an image surface, the light amount irregularity between light sources, and a reciprocity failure.
Even when the combinations of the pixel forming light emitting portions are plural, the same combination is periodically repeated. For this reason, the image density change may be periodically generated and an abnormal image called “banding” or “texture” may be easily generated.
In particular, when the number of light beams in multiple beams increases, a period of the image density change increases, and easily becomes a period of a region (about 1 line/mm) where perceptual sensitivity of human eyes is high. In this case, the banding is easily viewed.
As generation factors of the banding in an optical scanning device to perform optical scanning using multiple beams, (1) the change of the exposure width due to the deviation of the beam pitch, (2) the change of exposure distribution due to the light amount difference, (3) a reciprocity failure, and (4) the rotational change of a photosensitive drum or the mechanical change such as the surface tilt of a polygon mirror are known.
1. Regarding Deviation of Beam Pitch
FIG. 26A shows a state where the beam pitch is ideally adjusted. For example, when the density of the sub-pixels is 4800 dpi, light emitting portions are aligned at almost the interval of 5 μm on an image surface.
FIG. 26B shows the case where magnification of an optical system is deviated from an ideal value. In this case, even though plural light emitting portions are aligned at an equivalent interval, an interval of light beams is deviated from an ideal interval (5 μm). In FIG. 26B, the interval becomes wider than the ideal interval.
FIG. 26C shows the case where rotation of a light source around a central axis thereof is deviated. In this case, an interval of the plural light emitting portions in a sub-scanning direction is not constant.
As such, when one pixel is formed by plural light beams having the deviated beam pitch, a shape of one pixel is collapsed and the density is changed. Since the combinations of the pixel forming light emitting portions are sequentially changed and the same combination is periodically repeated, the banding is easily generated.
FIGS. 27(A) to 27(C) show influences from the deviation of the beam pitch in the case where two light beams (bm1 and bm2) and two light beams (bm3 and bm4) are deflected on different deflected reflection surfaces of a polygon mirror. FIGS. 27(A) to 27(C) show an aspect where the magnitude of one pixel changes according to the deviation of the beam pitch, in the case where one pixel is formed by 4 light beams. FIG. 27(A) shows the magnitude of one pixel when the beam pitch is the ideal pitch. FIG. 27(B) shows the magnitude of one pixel when the beam pitch of the light beams bm2 and bm3 is less than the ideal pitch. FIG. 27(C) shows the magnitude of one pixel when the beam pitch of the light beams bm2 and bm3 is more than the ideal pitch.
2. Regarding Light Amount Difference
FIG. 28(A) shows the case where the light amounts of 4 light beams are equal to each other. FIG. 28(B) shows the case where the light amounts of the light beams bm1 and bm4 are less than the light amounts in the case of FIG. 28(A) and the light amounts of the light beams bm2 and bm3 are more than the light amounts in the case of FIG. 28(A). In this case, an exposure range based on the four light beams in the sub-scanning direction becomes narrower than an exposure range in the case of FIG. 28(A).
FIG. 28(C) shows the case where the light amounts of the light beams bm1 and bm4 are more than the light amounts in the case of FIG. 28(A) and the light amounts of the light beams bm2 and bm3 are less than the light amounts in the case of FIG. 28(A). In this case, an exposure range based on the four light beams in the sub-scanning direction becomes wider than an exposure range in the case of FIG. 28(A).
Since the density of one pixel is periodically changed, this becomes a generation factor of the banding, similar to the deviation of the beam pitch.
Meanwhile, the optical scanning device executes so-called auto power control (APC) that monitors the light emission amount of the light emitting portion and controls a supplied current (or applied voltage) to cause the light emission amount to become the desired constant light amount. However, when the light source is a multiple beam light source having plural light emitting portions, even though the light emission amounts between the light emitting portions are constant, an optical path of each light beam that passes through an optical system is different. As a result, the light amount difference may be generated between light spots on the photosensitive drum, due to the transmittance change in optical components and birefringence in the optical components.
3. Regarding Reciprocity Failure
The reciprocity failure means a phenomenon of “reciprocity” where “a sensitivity characteristic of a photoreceptor is the same when exposure energy (=light amount×exposure time) is constant being not realized.
When the exposure energy is constant, if the light amount increases, the sensitivity (latent image depth) decreases. This is generated in the case where, if the light amount increases, the recombination amount of carriers increases and the amount of carriers reaching a surface decreases.
As shown in FIG. 29, when the photosensitive drum is collectively exposed by the four light beams, if a place A to be simultaneously exposed and a place B to be exposed in a one scanning time are compared with each other, the exposure energy in the place A is the same as the exposure energy in the place B. However, since one-time scanning light amount in the place A is more than one-time scanning light amount in the place B, the sensitivity is lowered due to the reciprocity failure, as shown in FIG. 30.
In this case, the high density is generally easily generated in a joint (portion corresponding to the place B). In this case, whether the reciprocity failure is easily generated depends on a characteristic of a photoreceptor and the scanning speed.
FIG. 31(A) shows the case where the four light beams (bm1 to bm4) are deflected on the same deflected reflection surface of the polygon mirror and are simultaneously exposed. FIG. 31(B) shows the case where the two light beams (bm1 and bm2) and the two light beams (bm3 to bm4) are deflected on the different deflected reflection surfaces of the polygon mirror and are exposed at different times. In the case of FIG. 31(B), in a region S where the light beam bm2 and the light beam bm3 overlap each other, the density becomes higher than the density of the other overlapping region.
Therefore, in regards to the sub-scanning direction, the exposure range based on the four light beams in the case of FIG. 31(B) becomes narrower than that in the case of FIG. 31(A). As a result, the density of one pixel is periodically changed according to scanning and this becomes a generation factor of the banding.
For this reason, technologies for decreasing the density difference between the scanning lines based on the reciprocity failure are suggested (for example, see Japanese Patent Application Laid-open No. 2002-113903 and Japanese Patent Application Laid-open No. 2007-196460).
Further, Japanese Patent Application Laid-open No. 2008-170640, Japanese Patent Application Laid-open No. H10-221903, Japanese Patent Application Laid-open No. 2000-037904, and Japanese Patent Application Laid-open No. 2009-029115 disclose technologies for decreasing banding.
However, in a latent image forming apparatus that is disclosed in Japanese Patent Application Laid-open No. 2002-113903 and a scanning exposure device that is disclosed in Japanese Patent Application Laid-open No. 2007-196460, the banding that is generated due to the deviation of the beam pitch is not considered.
In image forming apparatuses that are disclosed in Japanese Patent Application Laid-open No. 2008-170640 and Japanese Patent Application Laid-open No. H10-221903, the banding that is generated due to the deviation of the beam pitch is not considered.
In an image recording apparatus that disclosed in Japanese Patent Application Laid-open No. 2000-037904, banding in the case where the number of channels of laser beams is not a common multiple of the number of unit pixels of pattern dither cannot be decreased.
In an image forming apparatus that is disclosed in Japanese Patent Application Laid-open No. 2009-029115, the banding that is generated due to the deviation of the beam pitch is not considered.
According to the present invention, when one pixel is formed by at least two light beams, banding that is generated due to the deviation of the beam pitch can be decreased.
According to the present invention, since the image forming apparatus includes the optical scanning device, a high-quality image can be formed at a high speed.